The update_dimensions function in libavcodec/vp8.c in FFmpeg through 2.8.1, as used in Google Chrome before 46.0.2490.71 and other products, relies on a coefficient-partition count during multi-threaded operation, which allows remote attackers to cause a denial of service (race condition and memory corruption) or possibly have unspecified other impact via a crafted WebM file.
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.